192 ON THE INTERNAL FORM AND [ch. 



diameter across which the thread Hes, recede from the thread, 

 so forming a notch at each end of the diameter, while violent 

 streaming motions are set up at the surface, away from the thread 

 in the direction of the two opposite poles. Robertson* suggests, 

 accordingly, that the division of the cell is actually brought about 

 by a lowering of the equatorial surface-tension, and that this in 

 turn is due to a chemical action, such as a liberation of choUn, 

 or of soaps of cholin, through the splitting of lecithin in nuclear 

 synthesis. 



But purely chemical changes are not of necessity the funda- 

 mental cause of alteration in the surface-tension of the egg, for 

 the action of electrolytes on surface-tension is now well known 

 and easily demonstrated. So, according to other views than 

 those with which we have been dealing, electrical charges are 

 sufficient in themselves to account for alterations of surface- 

 tension ; while these in turn account for that protoplasmic 

 streaming which, as so many investigators agree, initiates the 

 segmentation of the egg|. A great part of our difficulty arises 

 from the fact that in such a case as this the various phenomena 

 are so entangled and apparently concurrent that it is hard to say 

 which initiates another, and to which this or that secondary 

 phenomenon may be considered due. Of recent years the pheno- 

 menon of adsorptio7L has been adduced (as we have already briefly 

 said) in order to account for many of the events and appearances 

 which are associated with the asymmetry, and lead towards the 

 division, of the cell. But our short discussion of this phenomenon 

 may be reserved for another chapter. 



Hov/ever, we are not directly concerned here with the 

 phenomena of segmentation or cell division in themselves, except 

 only in so far as visible changes of form are capable of easy and 

 obvious correlation with the play of force. The very fact of 

 "development" indicates that, while it lasts, the equihbrium of 

 the egg is never complete J. And we may simply conclude the 



* Robertson, T. B., Note on the Chemical Mechanics of Cell Division, Arclu 

 /. Entw. Mech. xxvn, p. 29, 1909, xxxv, p. 692. 1913. Cf. R. S. Lilhe, J. Exp. 

 Zool. XXI, pp. 369—402, 1916. 



t Cf. D'Arsonval, Arch, de Physiol, p. 460, 1889; Ida H. Hyde, op.cit. p. 242. 



I Cf. Plateau's remarks {Statique des liquides, n, p. 154) on the tendency towarda 

 equilibrium, rather than actual equilibrium, in many of his systems of soap-films^ 



